Electron-optical lens



Dec. 22, I942.

ELECTRONOPTICAL LENS Filed March 7, 1939 (3011 A!" Mi W M B. v. BORRIES ET AL 2,305,751

Patented Dec. 22, 1942 ELECTRON-OPTICAL LENS Bodo v. Borries and Ernst Ruska, Berlin-Spandau, Germany; vested in the Alien Property Custodian Application March '7, 1939, Serial No. 260,296 In Germany March '7, 1938 7 Claims.

The invention relates to electron-optical devices and more in particular to magnetic lenses of small focal length to be employed for magnifying or reducing images produced by electron rays.

It is well known to influence the electron rays in electron-optical instruments with the aid of magnet coils. In some cases, particularly in magnetic electron microscopes, it is necessary to avoid voltage fluctuations of the power sources energizing the magnet coils, as such fluctuations represent one of the faults which must be eliminated in order to obtain a higher resolving power than attainable in a light-optical microscope. It has been proposed to employ regulating devices for maintaining the energizing voltage of the magnetic lens systems of an electron microscope at a constant value; the result, however, is not satisfactory.

As is well known, the resolving power of electron microscopes may be increased considerably by providing objectives of very small focal length. For instance, the aperture fault as well as the chromatic aberration are proportional to the focal length so that magnetic systems with which relatively small focal lengths cannot be attained are not suitable for electron microscopes. This is the reason why heretofore permanent magnets have not been used for electron-optical magnifying or reducing lenses. It was believed, in particular, that when using permanent magnets the necessary small focal length could not be attained, and it was generally assumed that when using permanent magnets the magnet field could not be controlled to a sufiicient extent.

Our invention is based on the discovery that these prejudices are unjustified, and that it is readily possible to avoid the above-mentioned drawbacks of the known electron-optical lens systems by employing a particular lens structure comprising permanent magnets in combination with ferromagnetic pole pieces, which will be described in the following in conjunction with the drawing showing in Fig. 1 a perspective and sectional view of a lens system according to the invention, and in Fig. 2 a top View of the same embodiment. Fig. 3 shows an axial cross section of a second embodiment. Fig. 4 illustrates a third possibility by an axial cross section showing only one-half of the symmetrical structure. Figs. 5 and 6 represent a sectional side elevation and a top view, respectively, of a further example which, in general, is similar to that of Fig. 4.

The electron-optical lens structure shown in Fig. 1 contains two ferromagnetic pole pieces in the form of circular plates. The plates are rigid- 1y connected with each other and held in spaced relationship by a tubular supporting member 3 which consists of non-magnetic material, for instance brass, and forms a wall portion of the vacuum vesselof the electron-optical instrument containing the lens structure. The two pole plates 2 have a central portion ll provided with an aperture 5 for the passage of the electron rays. The two central portions are funnelshaped and project inwardly so that the perforated centers closely approach each other. Several permanent magnets l are arranged along the periphery of the plates outside the vacuum wall 3. By providing the plates 2 with openings 4, the advantage is obtained over the known magnet coils that when exhausting the vacuum vessel not only the aperture 5 but a considerably greater cross-section is available for the passage of the gas.

A coarse regulation of the strength of the magnetic field effective in the air gap may be effected by varying the number of the permanent magnets employed. To this end, one or more of the magnets I may be designed to be easily removable. An example of a device for effecting a fine regulation is shown in Fig. 21. According to the invention one or more of the permanent magnets may be movably arranged so that their contacts with the plates 2 may be varied as indicated by a dot-and-dash line, for instance by means of a screw 6 or another adjusting device. It is advisable to use an adjusting device having sufiiciently long lever arms in order to attain the desired sensitiveness. If, for instance, the length of the magnet is correspondingly increased or themagnet is provided with an extension which may be employed as a lever when regulating, the strength of the magnetic field may be varied in accordance with any desired sensitiveness. The same effect may be obtained by movably arranging the permanent magnets in such a manner that they may be displaced in parallel relation to the outer surface of the lens structure. It has been found convenient to design two or three of the magnets employed in the above-described manner. The range of regulation thus obtained is, as a rule, sufficient in view of the fact that for effecting a coarse regulation some of the magnets may be removed from the lens.

In the embodiment shown in Fig. 3 the permanent magnets I are arranged at the front faces of the cylindrical lens structure. The outer peripheral surface of the lens is formed by a portion of the metal wall I of the electron-optical instrument. In order to protect the electron ray passing through the aperture 5 against disturbing interferences caused by extraneous magnetic fields, annular inserts are provided as indicated at 9. They consist of a ferro-magnetic material, for instance iron.

In the embodiment shown in Fig. 4-. permanent magnets l are arranged in axial direction at the outer peripheral surface as well as radially at the front faces of the lens structure. Rings l0 serve to close the magnetic circuit between the magnets. 6 denotes a spacing member consisting of a vacuum wall of non-magnetic material. 1 denotes the wall proper of the electron-optical instrument and 3 the pole pieces. By the use of horse-shoe magnets the same effect may be obtained as by the arrangement of the magnets shown in Fig. 4.

In the embodiment shown in Fig. 4 the radially arranged magnets i should be sector shaped in order to arrange as great a number of magnets as possible in the manner shown. In some cases it may be preferable to render the crosssection of the radially arranged magnets uniform throughout their lengths in order to create the necessary flux without thereby varying the flux density. This is shown, for instance, in Fig. 5 in which I denotes the magnets and 8 the pole pieces. In this case the ends of the permanent magnets near the axis are narrow and high, whereas the opposite ends thereof are relatively broad and fiat. Particularly when designing the lens according to the invention in the manner as shown in Figs. i and 5 a very high magnetomotive force may be obtained with a relatively small height of the entire arrangement.

Preferably cobalt magnets or also the ironnickel-aluminum alloy known under the tradename Oerstite are employed in the manufacture of permanent magnets to be used according to the invention.

Electron-optical lenses according to the invention are simpler and more economical in construction and manufacture, and less space-consuming than the known electromagnetic lens systems. Lenses according to the invention further eliminate faults due to voltage fluctuations or other irregularities of electrical arrangements and do not require an energy supply when in operation. The magnetic lenses according to the invention may not only be employed for influencing electron rays but also for other corpuscular rays, for instance ion or proton rays.

What is claimed is:

1. In combination with an electron microscope having a vacuum vessel, a magnetic lens of small focal length comprising two annular pole plates of high magnetic permeability arranged in said vacuum vessel coaxially with the electron-optical axis and spaced from each other in the direction of the electron-optical axis in fixed relation to each other, each of said plates being provided with a centrally aperture pole piece approaching the pole piece of said other plate, and permanent magnet bodies forming the magnetomotive source of the lens and being arranged in parallel to said axis and around said pole piece to form a substantially cylindrical arrangement of magnets, said magnet bodies being connected with said plates to form a closed magnetic circuit between said two pole pieces and including a magnet body movable relative to said plates, and means for adjusting said movable body in order to regulate the magnetic field effective between said pole pieces.

2. In an electron microscope having a vacuum vessel, an electron-optical lens of small focal length comprising two circular disc-like plates of highly permeable magnetic material arranged in said vessel concentrically to the electron-optical axis and spaced from each other along said axis in fixed relation to each other, two centrally apertured pole pieces of highly permeable magnetic material arranged centrally with respect to said plates and being magnetically integral therewith respectively, said pole pieces facing each other to forma lens gap along and close to said axis, and permanent magnet means arranged at the periphery of said plates to form a substantially cylindrical arrangement and connecting said plates magnetically in order to energize the gap between said pole pieces.

3. In an electron microscope having a vacuum vessel, an electron-optical lens or" small focal length comprising two circular plates of highly permeable magnetic material arranged in said vessel concentrically to the electron-optical axis and. spaced from each other along said axis in fixed relation to each other, two centrally apertured pole pieces of highly permeable magnetic material arranged centrally with respect to said plates and being magnetically integral therewith respectively, said pole pieces facing each other to form a lens gap along and close to said axis, and a plurality of permanent magnet bars disposed round the periphery of said pole members and in parallel to said axis so as to form a substantially cylindrical magnet arrangement, said bars being magnetically connected with said plates to energize the gap between said pole pieces.

4. In an electron microscope having a vacuum vessel and a magnetic lens structure, said lens structure comprising two substantially circular and axially symmetrical pole plates of magnetically-soft material spaced from each other along the optical axis of the microscope, said plates being arranged in said vacuum vessel and having their central portions, respectively, provided with an axial aperture and shaped to approach each other to form a lens gap along and close to said axis, and permanent magnet means forming a magnetic circuit with said lens gap, said means including permanent magnet bodies disposed in parallel to said axis so as to form a substantially cylindrical arrangement around said plates, and other magnet bodies extending radially to said axis between said former magnet bodies and said plates for energizing said pole plates.

5. In an electron microscope having a vacuumtight enclosure formed in part of a pair of annular magnetic members positioned in mutually spaced end-to-end relation to provide a lens gap between them and tapered conically towards each other, magnetic means associated with said members for energizing said lens gap, and peripheral means externally sealing said gap to maintain the vacuum-tight qualities of the enclosure.

6. In an electron microscope having a vacuumtight enclosure formed in part of a pair of annular magnetic members positioned in mutually spaced end-to-end relation to provide a lens gap between them and tapered conically towards each other, magnetic means externally connected between said members for energizing said lens gap, and non-magnetic means peripherally sealing the interstice between said members to maintain said enclosure vacuum-tight.

7. In an electron microscope having a vacuumtight enclosure formed in part of a pair of annular magnetic members positioned in mutually spaced end-to-end relation to provide a lens gap between them and tapered conically towards each other, magnetic means associated with said members for energizing said lens gap, and a ring of non-magnetic material interposed between said members for peripherally sealing the interstice between said members.

BODO v. BORRI S. ERNST RUSKA. 

